Pressure Drop In Pipe Lines And Fittings | Part 1

April 21, 2021
Pressure plays a prominent role in driving and arresting the fluid flow from one place to another.

What Pressure Is Involved In Driving Fluid Through Pipes And Fittings?

One classical definition is

Pressure is force acted per unit area

Pressure in a pipeline may be due to pumping, vaporization, compression, etc. The fluid should travel the entire pipeline without losing its pressure otherwise we need to spend extra for pumping the fluid to compromise for the loss in pressure.

Why Is Pressure Lost Through Pipes?

The reason is due to friction, wake formation, separation of the boundary layer by fittings, pipe roughness, etc. In order for the pump to work efficiently, the pump will be designed to accommodate any extra pressure that is required.

Pressure drop due to unseparated boundary layers (Skin friction) can be calculated by the following classical formula,

Pressure drop by
Pressure drop varies with length and diameter of the pipe, velocity, and density of the fluid, and Fanning friction factor. Don’t confuse this fanning friction factor with Darcy’s friction factor.
The Fanning friction factor varies with the nature of the flow. So if we know the nature of flow i.e., laminar or turbulent we can calculate the ‘f ‘ value accordingly.
Reynolds number by
If the flow is laminar the fanning friction varies with Reynolds number as follows,
Friction factor by
If the flow is turbulent a lot of equations are in practice and you can use any of those equations at the expense of accuracy. the equations are mentioned as follows.
  • Colebrook equation (1938)
Colebrook equation from
  • Swamee and Jain equation (1976)
Swamee and jain equation from
  • Haaland equation. (1983)
Haaland equation from
The ‘ ε ‘ symbol corresponds to the roughness factor which depends on the material of the construction of the pipe, whereas D and Re have their usual meaning.
After calculating the friction factor, we can find the pressure drop due to skin friction in a pipeline by substituting the ‘ f ‘ value into the pressure drop formula stated earlier.
In the Second Part pressure drop due to fittings will be covered.

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